Catalytic articles

ABSTRACT

Catalytic articles comprising a substrate having a catalytic coating thereon, the catalytic coating comprising a catalytic layer having a thickness and an inner surface proximate to the substrate and an outer surface distal to the substrate; where the catalytic layer comprises a noble metal component on support particles and where the concentration of the noble metal component towards the outer surface is greater than the concentration towards the inner surface are highly effective towards treating exhaust gas streams of internal combustion engines. The articles are prepared via a method comprising providing a first mixture comprising micron-scaled support particles and applying the first mixture to a substrate to form a micro-particle layer; providing a second mixture comprising nano-scaled support particles and a noble metal component having an initial pH and applying the second mixture to the micro-particle layer and calcining the substrate.

The present invention is aimed at catalytic articles for use in treatingexhaust of an internal combustion engine.

BACKGROUND

Exhaust gas streams of internal combustion engines contain pollutantssuch as hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides(NOx) that foul the air. Catalysts useful in treating exhaust gases ofinternal combustion engines include platinum group metals (PGM), forinstance via oxidation of hydrocarbons and carbon monoxide.

There exists a need for still more efficient catalysts for the treatmentof exhaust gases of internal combustion engines.

SUMMARY

Accordingly, disclosed is a catalytic article comprising a substratehaving a catalytic coating thereon, the catalytic coating comprising acatalytic layer having a thickness and an inner surface proximate to thesubstrate and an outer surface distal to the substrate; where thecatalytic layer comprises a noble metal component on support particlesand where the concentration of the noble metal component towards theouter surface is greater than the concentration towards the innersurface.

Also disclosed is a method of making the catalytic articles comprisingproviding a first mixture comprising micron-scaled support particles andapplying the first mixture to a substrate to form a micro-particlelayer; providing a second mixture comprising nano-scaled supportparticles and a noble metal component having an initial pH and applyingthe second mixture to the micro-particle layer and calcining thesubstrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a SEM image of the inventive sample of Example 1. The arrowpoints to a monolith wall.

FIG. 2 is a graph of test results of CO conversion of a gas stream ofExample 1.

DETAILED DISCLOSURE

The present catalytic layer comprises a noble metal component on supportparticles. The noble metal is in particular a platinum group metal(PGM), for instance platinum or palladium. The catalytic coating layerhas a thickness, an inner surface proximate to a substrate and an outersurface distal to the substrate. The outer surface will face theatmosphere and/or exhaust gas stream of an engine. A platinum groupmetal component may comprise a mixture of platinum and palladium, forinstance at a weight ratio of from about 1:5 to about 5:1.

The noble metal is present in the catalytic layer in a gradient, thatis, the concentration of the noble metal towards the outer surface isgreater than the concentration towards the inner surface. The catalyticlayer thickness for instance may be from about 6, about 8 or about 10microns to about 15, about 20, about 30, about 50, about 75, about 100,about 150, about 200, about 250, about 300 or about 350 microns.

For instance, at least 50 wt % (weight percent) of the noble metalcomponent may reside in the outer one fifth (20%) of the thickness ofthe catalytic layer. Also, at least 60 wt % or at least 70 wt % of thenoble metal component may reside in the outer one half of the thicknessof the catalytic layer. For example from about 80 wt % to about 90 wt %of the noble metal may reside in the outer 20% of the thickness of thecatalytic layer.

The support for example comprises refractory metal oxides, which porousmetal-containing oxide materials exhibit chemical and physical stabilityat high temperatures, such as the temperatures associated with gasolineor diesel engine exhaust. Exemplary metal oxides include alumina,silica, zirconia, titania, ceria, praseodymia, tin oxide, and the like,as well as physical mixtures or chemical combinations thereof, includingatomically-doped combinations and including high surface area oractivated compounds such as activated alumina.

Included are combinations of metal oxides such as silica-alumina,ceria-zirconia, praseodymia-ceria, alumina-zirconia,alumina-ceria-zirconia, lanthana-alumina, lanthana-zirconia-alumina,baria-alumina, baria-lanthana-alumina, baria-lanthana-neodymia aluminaand alumina-ceria. Exemplary aluminas include large pore boehmite,gamma-alumina, and delta/theta alumina. Useful commercial aluminas usedas starting materials in exemplary processes include activated aluminas,such as high bulk density gamma-alumina, low or medium bulk densitylarge pore gamma-alumina and low bulk density large pore boehmite andgamma-alumina.

High surface area metal oxide supports, such as alumina supportmaterials, also referred to as “gamma alumina” or “activated alumina,”typically exhibit a BET surface area in excess of 60 m²/g, often up toabout 200 m²/g or higher. An exemplary refractory metal oxide compriseshigh surface area γ-alumina having a specific surface area of about 50to about 300 m²/g. Such activated alumina is usually a mixture of thegamma and delta phases of alumina, but may also contain substantialamounts of eta, kappa and theta alumina phases. “BET surface area” hasits usual meaning of referring to the Brunauer, Emmett, Teller methodfor determining surface area by N₂ adsorption. Desirably, the activealumina has a specific surface area of about 60 to about 350 m²/g, forexample from about 90 to about 250 m²/g.

In certain embodiments, metal oxide supports useful in the catalystcompositions disclosed herein are doped alumina materials, such asSi-doped alumina materials (including, but not limited to 1-10%SiO₂—Al₂O₃), doped titania materials, such as Si-doped titania materials(including, but not limited to 1-10% SiO₂—TiO₂), or doped zirconiamaterials, such as Si-doped ZrO₂ (including, but not limited to 5-30%SiO₂—ZrO₂).

Advantageously, a refractory metal oxide may be doped with one or moreadditional metal oxide dopants, such as lanthana, baria, strontiumoxide, calcium oxide, magnesium oxide, or combinations thereof. Themetal oxide dopant is typically present in an amount of about 1 to about20% by weight, based on the weight of the catalytic layer.

The dopant metal oxides can be introduced using an incipient wetnessimpregnation technique or through usage of colloidal mixed oxideparticles. Preferred dopant metal oxides include colloidalbaria-alumina, baria-zirconia, baria-titania, zirconia-alumina,baria-zirconia-alumina, lanthana-zirconia and the like.

Thus the refractory metal oxides or refractory mixed metal oxides in thecatalytic layer are most typically selected from the group consisting ofalumina, zirconia, silica, titania, ceria, for example bulk ceria,manganese oxide, zirconia-alumina, ceria-zirconia, ceria-alumina,lanthana-alumina, baria-alumina, silica, silica-alumina and combinationsthereof. These refractory metal oxides in the catalytic layer may befurther doped with base metal oxides such as baria-alumina,baria-zirconia, baria-titania, zirconia-alumina, baria-zirconia-alumina,lanthana-zirconia and the like.

The catalytic layer may comprise any of the above named refractory metaloxides and in any amount. For example the refractory metal oxides in thecatalytic layer may comprise at least about 15, at least about 20, atleast about 25, at least about 30 or at least about 35 wt % (weight %)alumina where the wt % is based on the total dry weight of the catalyticlayer. The catalytic layer may for example comprise from about 15 toabout 95 wt % alumina or from about 20 to about 85 wt % alumina.

The catalytic layer comprises for example from about 15 wt %, about 20wt %, about 25 wt %, about 30 wt % or about 35 wt % to about 50 wt %,about 55 wt %, about 60 wt % about 65 wt % or about 70 wt % aluminabased on the weight of the catalytic layer.

Advantageously, the catalytic layer may comprise ceria, alumina andzirconia.

The noble metal is for example present in the catalytic layer from about0.1 wt %, about 0.5 wt %, about 1.0 wt %, about 1.5 wt % or about 2.0 wt% to about 3 wt %, about 5 wt %, about 7 wt %, about 9 wt %, about 10 wt%, about 12 wt % or about 15 wt %, based on the weight of the layer.

The noble metal is for example present from about 2 g/ft³, about 5g/ft³, about 10 g/ft³, about 15 g/ft³ or about 20 g/ft³ to about 40g/ft³, about 50 g/ft³, about 60 g/ft³, about 70 g/ft³, about 80 g/ft³,about 90 g/ft³ or about 100 g/ft³, based on the volume of the substrate.

The catalytic layer in addition to the refractory metal oxide and PGMmay further comprise any one or combinations of the oxides of lanthanum,barium, praseodymium, neodymium, samarium, strontium, calcium,magnesium, niobium, hafnium, gadolinium, manganese, iron, tin, zinc orcopper.

The oxygen storage component (OSC) is an entity that has multi-valentoxidation states and can actively react with oxidants such as oxygen(O₂) or nitric oxides (NO₂) under oxidative conditions or react withreductants such as carbon monoxide (CO), hydrocarbons (HC) or hydrogen(H₂) under reduction conditions. Examples of suitable oxygen storagecomponents include ceria and praseodymia. An OSC is sometimes used inthe form of mixed oxides. For example, ceria can be delivered as a mixedoxide of cerium and zirconium and/or a mixed oxide of cerium, zirconiumand neodymium. For example, praseodymia can be delivered as a mixedoxide of praseodymium and zirconium and/or a mixed oxide ofpraseodymium, cerium, lanthanum, yttrium, zirconium and neodymium.

For instance, OSC components are metal oxides and/or mixed metal oxidesof metals selected from the group consisting of cerium, zirconium,neodymium, praseodymia, lanthanum and yttrium.

An OSC component may be present in the catalytic layer for example fromabout 1 wt % to about 65 wt %. For example, an OSC component such asceria may be present from about 1 wt % to about 60 wt %, from about 5 toabout 50 wt % or from about 8 to about 40 wt % of the total dry weightof the layer.

The catalytic layer may further comprise a base metal oxide for examplean oxide of lanthanum, barium, praseodymium, neodymium, samarium,strontium, calcium, magnesium, niobium, hafnium, gadolinium, manganese,iron, tin, zinc, copper or combinations thereof. Base metal oxides maybe present from about 0.1 to about 5.0 wt %, based on the total dryweight of the layer.

The present catalytic layer advantageously contains support particleshaving a bimodal particle size distribution comprising micron-scaledparticles and nano-scaled particles.

Micron-scaled particles for example have an average particle size fromabout 1, about 2, about 3, about 4 or about 5 microns to about 6, about7, about 8, about 9, about 10 or about 11 microns. For example, presentmicron-scaled particles have a D90 of from about 8, about 9 or about 10microns to about 12, about 13, about 14, about 15 microns, about 20,about 25, about 30, about 35, about 40 or about 50 microns.

The nano-scaled particles for instance have an average particle size of≤950 nm, ≤900 nm, ≤850 nm, ≤800 nm, ≤750 nm, ≤700 nm, ≤650 nm, ≤600 nm,≤550 nm, ≤500 nm, ≤450 nm, ≤400 nm, ≤350 nm, ≤300 nm, ≤250 nm, ≤200 nm,≤150 nm or ≤100 nm. For example, having an average particle size of fromabout 1 nm, about 2 nm, about 3 nm, about 4 nm, about 5 nm, about 7 nm,about 10 nm, about 15 nm, about 20 nm, about 25 nm, about 30 nm, about35 nm, about 40 nm, about 45 nm or about 50 nm to about 200 nm, about300 nm, about 400 nm, about 500 nm, about 600 nm, about 700 nm, about800 nm or about 900 nm.

Oxygen storage components in the present invention are considered to bea possible support particle, together with other supports such asalumina or on their own. That is, the discussion regarding particle sizerefers also to oxygen storage components.

Particles may be primary particles and/or may be in the form ofagglomerates. Particle size refers to primary particles.

The term “substrate” refers in general to a monolithic material ontowhich a catalytic coating is disposed, for example a flow-throughmonolith or monolithic wall-flow filter. In one or more embodiments, thesubstrate is a ceramic or metal having a honeycomb structure. Anysuitable substrate may be employed, such as a monolithic substrate ofthe type having fine, parallel gas flow passages extending from an inletend to an outlet end of the substrate such that passages are open tofluid flow. The passages, which are essentially straight paths fromtheir fluid inlet to their fluid outlet, are defined by walls on which acatalytic coating is disposed so that gases flowing through the passagescontact the catalytic material. The flow passages of the monolithicsubstrate are thin-walled channels, which can be of any suitablecross-sectional shape and size such as trapezoidal, rectangular, square,sinusoidal, hexagonal, oval, circular, etc. Such structures may containfrom about 16 to about 900 or more gas inlet openings (i.e. cells) persquare inch of cross-section.

Present substrates are 3-dimensional having a length and a diameter anda volume, similar to a cylinder. The shape does not necessarily have toconform to a cylinder. The length is an axial length defined by an inletend and an outlet end.

The inlet end of a substrate is synonymous with the “upstream” end or“front” end. The outlet end is synonymous with the “downstream” end or“rear” end. A substrate will have a length and a width and a volume. Anupstream zone is upstream of a downstream zone. A zone of a catalyzedsubstrate is defined as a cross-section having a certain coatingstructure thereon.

Flow-through monolith substrates for example have a volume of from about50 in³ to about 1200 in³, a cell density of from about 16 cells persquare inch (cpsi) to about 500 cpsi or up to about 900 cpsi, forexample from about 200 to about 400 cpsi and a wall thickness of fromabout 50 to about 200 microns or about 400 microns.

The substrate may be a “flow-through” monolith as described above.Alternatively, a catalytic coating may be disposed on a wall-flow filtersoot filter, thus producing a Catalyzed Soot Filter (CSF). If awall-flow substrate is utilized, the resulting system will be able toremove particulate matter along with gaseous pollutants. The wall-flowfilter substrate can be made from materials commonly known in the art,such as cordierite, aluminum titanate or silicon carbide. Loading of thecatalytic coating on a wall-flow substrate will depend on substrateproperties such as porosity and wall thickness and typically will belower than the catalyst loading on a flow-through substrate.

Wall-flow filter substrates useful for supporting the SCR catalyticcoatings have a plurality of fine, substantially parallel gas flowpassages extending along the longitudinal axis of the substrate.Typically, each passage is blocked at one end of the substrate body,with alternate passages blocked at opposite end-faces. Such monolithiccarriers may contain up to about 700 or more flow passages (or “cells”)per square inch of cross-section, although far fewer may be used. Forexample, the typical carrier usually has from about 100 to about 300,cells per square inch (“cpsi”). The cells can have cross-sections thatare rectangular, square, circular, oval, triangular, hexagonal, or areof other polygonal shapes. Wall-flow substrates typically have a wallthickness from about 50 microns to about 500 microns, for example fromabout 150 microns to about 400 microns. Wall-flow filters will generallyhave a wall porosity of at least 40% with an average pore size of atleast 10 microns prior to disposition of the catalytic coating. Forinstance, wall-flow filters will have a wall porosity of from about 50to about 75% and an average pore size of from about 10 to about 30microns prior to disposition of a catalytic coating.

Catalyzed wall-flow filters are disclosed for instance in U.S. Pat. No.7,229,597. This reference teaches a method of applying a catalyticcoating such that the coating permeates the porous walls, that is, isdispersed throughout the walls. Flow-through and wall-flow substratesare also taught for example in U.S. Pat. app. No. 62/072,687, publishedas WO2016/070090.

For example, in the present systems the first substrate is a porouswall-flow filter and the second substrate is a flow-through monolith oralternatively, the first substrate is a flow-through monolith and thesecond substrate is a porous wall-flow filter. Alternatively, bothsubstrates may be identical and may be flow-through or wall-flowsubstrates.

The present catalytic coating may be on the wall surface and/or in thepores of the walls, that is “in” and/or “on” the filter walls. Thus, thephrase “having a catalytic coating thereon” means on any surface, forexample on a wall surface and/or on a pore surface.

The catalytic layer may each extend the entire length of the substrateor may extend a portion of the length of the substrate. The catalyticlayer may extend from either the inlet or outlet end. For example, thecatalytic layer may extend from the outlet end towards the inlet endabout 10%, about 20%, about 30%, about 40%, about 50%, about 60%, about70% or about 80% of the substrate length. Alternatively, the catalyticlayer may extend from the inlet end towards the outlet end about 10%,about 20%, about 30%, about 40%, about 50%, about 60%, about 70% orabout 80% of the substrate length.

The present catalytic coating may consist of the catalytic layer whichis in direct contact with the substrate and directly exposed to anexhaust gas stream. Alternatively, the catalytic coating may compriseone or more other coating layers besides the present catalytic layer.One or more “undercoats” may be present, so that at least a portion ofthe catalytic layer is not in direct contact with the substrate (butrather with the undercoat). One or more “overcoats” may also be present,so that at least a portion of the catalytic layer is not directlyexposed to a gaseous stream or atmosphere (but rather is in contact withthe overcoat). One or more interlayers may also be present.

An undercoat is a layer “under” a coating layer, an overcoat is a layer“over” a coating layer and an interlayer is a layer “between” twocoating layers.

The interlayer(s), undercoat(s) and overcoat(s) may contain one or morecatalysts or may be free of catalysts.

The internal combustion engine is for example a small engine, forinstance two-stroke or four-stroke spark ignition engines used toprovide power to machinery such as lawn mowers, chain saws, leafblowers, string cutters, motor scooters, motorcycles, mopeds and thelike. Small engines produce exhaust gas streams having a highconcentration of unburned fuel and unconsumed oxygen.

The method of forming the present graded catalytic layers comprisesproviding a first mixture comprising micron-scaled support particles andapplying the first mixture to a substrate to form a micro-particlelayer; providing a second mixture comprising nano-scaled supportparticles and a noble metal component and having an initial pH andapplying the second mixture to the micro-particle layer and calciningthe thus coated substrate.

The support particles of the first and second mixtures may have the sameor different chemical compositions. That is, they may be identical(other than having different average particle size). Alternatively, theymay have different chemical compositions. The support particles of eachof the first and second mixtures may comprise a refractory metal oxideparticles and/or oxygen storage component particles.

For example, a washcoat of finely divided micron-scaled particles of ahigh surface area refractory metal oxide such as gamma alumina areslurried in an appropriate vehicle, e.g. water. Along with the highsurface area refractory metal oxide, an oxygen storage component mayoptionally be included and slurried with the refractory metal oxide.This slurry or washcoat is applied to a substrate. The substrate may bedipped one or more times in such a slurry or the slurry may be coated onthe substrate such that there will be deposited on the substrate thedesired loading of the refractory metal oxide and optional oxygenstorage component.

The micron-scaled particles are advantageously milled to provide thedesired particle size range.

The coated substrate results in a micro-particle layer. Thereafter, thecoated substrate may advantageously be calcined by heating at atemperature from about 400° C. to about 600° C. for a period of fromabout 10 minutes to about 4 hours.

For instance, the first mixture contains micron-scaled ceria-aluminacomposite or micron-scaled ceria-alumina composite and micron-scaledbulk ceria.

Nanoparticles of a refractory metal oxide are treated with a noble metalcomponent to form the metal component deposited on and/or impregnated inthe refractory metal oxide nanoparticles. In this step also, refractorymetal oxide nanoparticles may be combined with oxygen storage componentnanoparticles. Alternatively, oxygen storage component nanoparticles aretreated with the noble metal component to form the metal componentdeposited on and/or impregnated in oxygen storage componentnanoparticles.

The mixture comprising nano-scaled particles may be in the form of a solor colloidal dispersion. The dispersion or sol will normally bedispersed in water and of a colloidal nature. A sol is a stabledispersion containing nano-scaled particles.

Advantageously, the mixture comprising nano-scaled particles is a sol.For instance, preparation of the second mixture comprises addition of azirconium sol and an aluminum sol or addition of a zirconium sol, analuminum sol and a cerium sol; and also addition of a suitable noblemetal compound or complex.

The noble metal components employed in the methods may be water-solublecompounds (e.g., precursor salts) or water-dispersible compounds(colloidal particles) or complexes. For example palladium compounds orcomplexes are typically used for deposition/impregnation. Generallyaqueous solutions of soluble compounds or complexes of a PGM componentare utilized. During the calcination step, or at least during theinitial phase of use of the composite, such compounds are converted intoa catalytically active form of the metal or a compound thereof.Generally, aqueous solutions of soluble compounds or complexes of theprecious metals are used such as a platinum group metal salt or acolloidal dispersion of a platinum group metal. For example acetatesalts, amine salts, nitrate salts, amine complex salts, nitrites,chlorides, bromides, iodides, sulfates of amine complex salts, diaminecomplex salts or tetraamine complex salts.

Specific palladium salts or complexes are for example palladium nitrate,palladium tetraamine hydroxide, colloidal palladium, palladium acetate,palladium nitrite, palladium diacetate, palladium(II) chloride,palladium(II) iodide, palladium (II) bromide, ammoniumhexachlor-palladate(IV), ammonium tetrachloro-palladate(II),palladium(II) oxide, palladium(II) sulfate,cis-diamminedichloro-palladium(II), diamminedinitro-palladium(II),hydrogen tetrachloro-palladate(II), potassium hexachlor-palladate(IV),potassium tetrachlor-palladate(II), sodium tetrachloro-palladate(II),tetraamine palladium(II) chloride and tetraamine palladium hydrogencarbonate; for example palladium nitrate, palladium tetraamine hydroxideor colloidal palladium.

The weight ratio of the solids of the second mixture to the solids ofthe first mixture is for example from about 1 to about 1, about 2, about3, about 4, about 5, about 6, about 7 or about 8.

The initial pH of the sol or colloidal dispersion of the second mixturecontaining nano-scaled particles may be ≥6 or ≥7. The nano-scaledmixture may advantageously be treated with an inorganic acid or anorganic acid which is believed to aid in deposition (fixing) of thenoble metal component onto the nanoparticles of refractory metal oxideand/or oxygen storage component.

The acid treatment of the second mixture may result in a pH adjustmentfor example to ≤6, ≤5, ≤4 or ≤3. For example, the acid treatment mayresult in a lower pH of from about 2, about 3 or about 4 to about 5,about 6, about 7, about 8, about 9, about 10, about 11 or about 12.

Inorganic acids include, but are not limited to, nitric acid.Dicarboxylic organic acids are especially effective in fixing PGM onsupport nanoparticles. Organic dicarboxylic acids include for exampleoxalic, malonic, succinic, glutamic, adipic, maleic, fumaric, phthalic,tartaric, pimelic acid, malic acid, sebacic acid, maleic acid, glutaricacid, azelaic acid, oxalic acid caccharic acid, aspartic acid, tartronicacid, mesoxalic acid, oxalacetic acid acetone dicarboxylic acid,itaconic acid, citric acid and the like.

For instance, a mixture containing nano-scaled particles may be preparedby mixing a cerium hydroxide sol, a zirconium nitrate sol, an aluminasol and a Pd (II) salt. A cerium sol contains for instance a cerium saltsuch as cerium hydroxide. A zirconium sol contains for instance azirconium salt such as zirconium nitrate.

The nano-scaled particle mixture may be pH adjusted to for example fromabout 4 to about 5 with an organic acid such as tartaric acid.Advantageously, barium and/or lanthanum salts are added such as bariumhydroxide and/or lanthanum hydroxide.

The weight ratio of the organic dicarboxylic acid to the noble metalcomponent is for example from about 6, about 5, about 4, about 3 orabout 2 to about 1.

The nano-scaled particles for instance have an average particle size of≤950 nm, ≤900 nm, ≤850 nm, ≤800 nm, ≤750 nm, ≤700 nm, ≤650 nm, ≤600 nm,≤550 nm, ≤500 nm, ≤450 nm, ≤400 nm, ≤350 nm, ≤300 nm, ≤250 nm, ≤200 nm,≤150 nm or ≤100 nm. For example, having an average particle size of fromabout 10 nm, about 15 nm, about 20 nm, about 25 nm, about 30 nm, about35 nm, about 40 nm, about 45 nm or about 50 nm to about 200 nm, about300 nm, about 400 nm, about 500 nm, about 600 nm, about 700 nm, about800 nm or about 900 nm.

The second mixture containing nano-scaled particles is applied to theformed micro-particle layer. The substrate having a micro-scaledparticle layer thereon may be dipped one or more times in the secondmixture containing nano-scaled particles to provide a desired loading ofthe refractory metal oxide, optional oxygen storage component and noblemetal component.

The degree of the noble metal gradient may be adjusted by the degree offixing of the noble metal on the support nanoparticles. The degree offixing may be adjusted by the amount of acid treatment. The greater thedegree of fixing, the more noble metal will be concentrated toward theouter surface of the catalytic coating.

The degree of fixing may be determined by directly measuring the amountof noble metal remaining in the supernatant fraction aftercentrifugation following the fixation step. For example, from about 40%,about 50% or about 60% to about 70%, about 80%, about 90%, about 95% orabout 99% of the noble metal, by weight, are fixed to the supportnanoparticles.

Stabilizers and/or promoters may also be incorporated into the firstand/or second mixtures, for example barium acetate and/or lanthanumnitrate.

The weight ratio of the solids of the first mixture to the solids of thesecond mixture are for instance from about 10:1, about 9:1, about 8:1,about 7:1, about 6:1, about 5:1, about 4:1, about 3:1, about 2:1 toabout 1:1.

The substrate now coated with both the first and second mixtures iscalcined, for example by heating at a temperature from about 400° C. toabout 600° C. for a period of from about 10 minutes to about 4 hours.

The present catalytic coating, as well as each zone of a catalyticcoating or any section of a coating, is present on the substrate at aloading (concentration) of for instance from about 0.2 g/in³ to about4.5 g/in³, or from about 0.4, about 0.5, about 0.6, about 0.7, about0.8, about 0.9 or about 1.0 g/in³ to about 1.5 g/in³, about 2.0 g/in³,about 2.5 g/in³, about 3.0 g/in³, about 3.5 g/in³ or about 4.0 g/in³based on the substrate. This refers to dry solids weight per volume ofsubstrate, for example per volume of a honeycomb monolith.

These methods supply the present gradient noble metal supportedcatalytic layer. The support particles advantageously have a bimodalparticle size distribution.

The present catalytic coating may function as an oxidation catalyst.

A treatment system contains one or more catalytic articles. A presentexhaust gas treatment system includes the present catalytic article andoptionally a further catalytic article. Further catalytic articlesinclude selective catalytic reduction (SCR) articles, diesel oxidationcatalysts (DOC), soot filters, ammonia oxidation catalysts (AMOx) andlean NOx traps (LNT).

The present treatment system may further comprise a selective catalyticreduction catalyst and/or diesel oxidation catalyst and/or a soot filterand/or an ammonia oxidation catalyst. A soot filter may be uncatalyzedor may be catalyzed (CSF).

“Noble metal components” refer to noble metals or compounds thereof,such as oxides. Noble metals are ruthenium, rhodium, palladium, silver,osmium, iridium, platinum and gold.

“Platinum group metal components” refer to platinum group metals orcompounds thereof, for example oxides. Platinum group metals areruthenium, rhodium, palladium, osmium, iridium and platinum.

Noble metal components and platinum group metal components also refer toany compound, complex, or the like which, upon calcinations or usethereof, decomposes or otherwise converts to a catalytically activeform, usually the metal or the metal oxide.

The term “exhaust stream” or “exhaust gas stream” refers to anycombination of flowing gas that may contain solid or liquid particulatematter. The stream comprises gaseous components which may containcertain non-gaseous components such as liquid droplets, solidparticulates and the like. An exhaust stream of an internal combustionengine typically further comprises combustion products, products ofincomplete combustion, oxides of nitrogen, combustible and/orcarbonaceous particulate matter (soot) and un-reacted oxygen and/ornitrogen.

“BET surface area” has its usual meaning of referring to theBrunauer-Emmett-Teller method for determining surface area byN₂-adsorption measurements. Unless otherwise stated, “surface area”refers to BET surface area.

D90 particle size distribution indicates that 90% of the particles (bynumber) have a Feret diameter below a certain size as measured byScanning Electron Microscopy (SEM) or Transmission Electron Microscopy(TEM). Average particle size is synonymous with D50, meaning half of thepopulation resides above this point, and half below. Particle sizerefers to primary particles. Particle size may be measured by laserlight scattering techniques, with dispersions or dry powders, forexample according to ASTM method D4464.

The articles “a” and “an” herein refer to one or to more than one (e.g.at least one) of the grammatical object. Any ranges cited herein areinclusive. The term “about” used throughout is used to describe andaccount for small fluctuations. For instance, “about” may mean thenumeric value may be modified by ±5%, ±4%, ±3%, ±2%, ±1%, ±0.5%, ±0.4%,±0.3%, ±0.2%, ±0.1% or ±0.05%. All numeric values are modified by theterm “about” whether or not explicitly indicated. Numeric valuesmodified by the term “about” include the specific identified value. Forexample “about 5.0” includes 5.0.

Unless otherwise indicated, all parts and percentages are by weight.Weight percent (wt %), if not otherwise indicated, is based on an entirecomposition free of any volatiles, that is, based on dry solids content.

All U.S. patent applications, published patent applications and patentsreferred to herein are hereby incorporated by reference.

Following are some embodiments of the invention.

E1. A catalytic article comprising a substrate having a catalyticcoating thereon,the catalytic coating comprising a catalytic layer having a thicknessand an inner surface proximate to the substrate and an outer surfacedistal to the substrate; wherethe catalytic layer comprises a noble metal component on supportparticles and where the concentration of the noble metal componenttowards the outer surface is greater than the concentration towards theinner surface.E2. A catalytic article according to embodiment 1 where at least 50 wt %of the noble metal component resides in the outer one fifth of thethickness of the catalytic layer.E3. A catalytic article according to embodiments 1 or 2 where at least60 wt % or at least 70 wt % of the noble metal component resides in theouter one half of the thickness of the catalytic layer.E4. A catalytic article according to any of the preceding embodimentswhere the noble metal is palladium or platinum.E5. A catalytic article according to any of the preceding embodimentswhere the support particles comprise a refractory metal oxide, forexample a refractory metal oxide selected from the group consisting ofalumina, zirconia, titania, ceria, manganese oxide, zirconia-alumina,ceria-zirconia, ceria-alumina, lanthana-alumina, baria-alumina, silica,silica-alumina and combinations thereof.E6. A catalytic article according to any of the preceding embodimentswhere the support particles comprise an oxygen storage component, forexample oxides of cerium, zirconium, neodymium, praseodymia, lanthanum,yttrium or combinations thereof.E7. A catalytic article according to any of the preceding embodimentswhere the catalytic layer comprises from about 15 wt %, about 20 wt %,about 25 wt %, about 30 wt % or about 35 wt % to about 50 wt %, about 55wt %, about 60 wt % about 65 wt % or about 70 wt % alumina based on theweight of the catalytic layer.E8. A catalytic article according to any of the preceding embodimentswhere the catalytic layer comprises ceria, alumina and zirconia.E9. A catalytic article according to any of the preceding embodimentswhere the noble metal is present in the catalytic layer from about 0.1wt %, about 0.5 wt %, about 1.0 wt %, about 1.5 wt % or about 2.0 wt %to about 3 wt %, about 5 wt %, about 7 wt %, about 9 wt %, about 10 wt%, about 12 wt % or about 15 wt %, based on the weight of the layer.E10. A catalytic article according to any of the preceding embodimentswhere the noble metal is present from about 2 g/ft³, about 5 g/ft³,about 10 g/ft³, about 15 g/ft³ or about 20 g/ft³ to about 40 g/ft³,about 50 g/ft³, about 60 g/ft³, about 70 g/ft³, about 80 g/ft³, about 90g/ft³ or about 100 g/ft³, based on the volume of the substrate.E11. A catalytic article according to any of the preceding embodimentswhere the catalytic layer contains support particles having a bimodalparticle size distribution comprising micron-scaled particles andnano-scaled particles, for example containing particles having anaverage particle size from about 1, about 2, about 3, about 4 or about 5microns to about 6, about 7, about 8, about 9, about 10 or about 11microns and particles having an average particle size of 950 nm, 900 nm,850 nm, 800 nm, 750 nm, 700 nm, 650 nm, 600 nm, 550 nm, 500 nm, 450 nm,400 nm, 350 nm, 300 nm, 250 nm, 200 nm, 150 nm or 100 nm.E12. A catalytic article according to any of the preceding embodimentswhere the substrate is a porous wall-flow filter or a flow-throughmonolith.E13. A catalytic article according to any of the preceding embodimentswhere the substrate is ceramic or metallic.E14. A catalytic article according to any of the preceding embodimentswhere 80 wt %, for example from about 80 wt % to about 90 wt % of thenoble metal resides in the outer 20% of the thickness of the catalyticlayer.E15. An exhaust gas treatment system comprising a catalytic articleaccording to any of the preceding embodiments in fluid communicationwith and downstream of an internal combustion engine.E16. A method of treating an exhaust stream of an internal combustionengine comprising contacting the exhaust stream with the catalyticarticle according to any of the preceding embodiments.

Following are some more embodiments.

E1. A method of making a catalytic article comprisingproviding a first mixture comprising micron-scaled support particles andapplying the first mixture to a substrate to form a micro-particlelayer;providing a second mixture comprising nano-scaled support particles anda noble metal component having an initial pH and applying the secondmixture to the micro-particle layer and calcining the substrate.E2. The method according to embodiment 1 where the micron-scaledparticles have an average particle size from about 1, about 2, about 3,about 4 or about 5 microns to about 6, about 7, about 8, about 9, about10 or about 11 microns and the nano-scaled particles have an averageparticle size of ≤950 nm, ≤900 nm, ≤850 nm, ≤800 nm, ≤750 nm, ≤700 nm,≤650 nm, ≤600 nm, ≤550 nm, ≤500 nm, ≤450 nm, ≤400 nm, ≤350 nm, ≤300 nm,≤250 nm, ≤200 nm, ≤150 nm or ≤100 nm.E3. The method according to embodiments 1 or 2 where the second mixtureis a sol or a colloidal dispersion.E4. The method according to any of the preceding embodiments where theweight ratio of the solids of the second mixture to the solids of thefirst mixture is from about 1 to about 1, about 2, about 3, about 4,about 5, about 6, about 7 or about 8.E5. The method according to any of the preceding embodiments where thesecond mixture is a sol having an initial pH ≥2, ≥3, ≥4, ≥5, ≥6, ≥7, ≥8,≥9 or ≥10.E6. The method according to any of the preceding embodiments comprisingadjusting the initial pH of the second mixture, for example adjustingthe pH to ≤6, ≤5, ≤4 or ≤3.E7. The method according to any of the preceding embodiments where thesecond mixture further comprises an organic dicarboxylic acid, forexample a dicarboxylic acid selected from the group consisting ofpimelic acid, fumaric acid, malic acid, adipic acid, sebacic acid,maleic acid, glutaric acid, azelaic acid, oxalic acid, tartaric acid,saccharic acid, aspartic acid, glutamic acid, tartronic acid, mesoxalicacid, oxaloacetic acid, adertone dicaroxylic acid and itaconic acid.E8. The method according to embodiment 7 where the weight ratio oforganic dicarboxylic acid to the noble metal component is from about 6,about 5, about 4, about 3 or about 2 to about 1.E9. The method according to any of the preceding embodiments where thefirst mixture further comprises micron-scaled oxygen storage componentparticles.E10. The method according to any of the preceding embodiments where thesecond mixture further comprises nano-scaled oxygen storage componentparticles.E11. The method according to any of the preceding embodiments where thesupport particles of the first and second mixtures have identicalchemical compositions.E12. The method according to any of the preceding embodiments where thesupport particles of the first and second mixtures have differentchemical compositions.E13. The method according to any of the preceding embodiments where thesupport particles comprise a refractory metal oxide, for example arefractory metal oxide selected from the group consisting of alumina,zirconia, titania, ceria, manganese oxide, zirconia-alumina,ceria-zirconia, ceria-alumina, lanthana-alumina, baria-alumina, silica,silica-alumina and combinations thereof.E14. The method according to any of the preceding embodiments where thesecond mixture is a sol.E15. The method according to any of the preceding embodiments where thesecond mixture comprises a zirconium sol and an aluminum sol.E16. The method according to any of the preceding embodiments where thesecond mixture comprises a zirconium sol, an aluminum sol and a ceriumsol.E17. A catalytic article prepared according to any of the aboveembodiments.

Example 1

A first mixture of CeO₂ and Al₂O₃ is combined with a nonionic surfactantand a protonated acid and distilled water and sufficiently mixed tocreate a homogenous dispersion. Mixing occurs over 20 minutes where thematerial undergoes particle size reduction to D90 14 microns, +/−3microns. Soluble cerium salt is then added with an amorphous aluminabinder and the pH is adjusted to 3.5 to 5. This mixture/dispersion isapplied to a support, dried and calcined at 500° C. for approximatelyone hour.

A second mixture of cerium and zirconium sols, colloidal alumina andpalladium salt is prepared and precipitated with a carboxylic acid.Following fixing of the Pd metal onto the sols, additional distilledwater, barium hydroxide, lanthanum nitrate solution and binders areadded and mixed for an additional 20 minutes.

The second mixture is applied over the first coating, dried and calcinedat 500° C. for approximately one hour. The top coat is applied at aloading of 0.25 g/in³ and the bottom coat is applied at a loading of 1.0g/in³. The total coating contains 0.70 g/in³ CeO₂, 0.46 g/in³ Al₂O₃,0.017 g/in³ Pd, 0.012 g/in³ La₂O₃, 0.0044 g/in³ Ba(OH)₂ and 0.044g/in³ZrO₂.

FIG. 1 is a SEM image of the inventive coating. The arrow is pointed tothe monolith wall. Pd is in adherence to CeO₂, La₂O₃, ZrO₂ and Al₂O₃particles. Zr and Ce particles are typically the bright or white, whileAl is a dull grey color.

The inventive coated monolith exhibits a high concentration of Pdtowards the coating surface exposed to the atmosphere.

The coated monolith is tested vs. a standard coating having welldispersed Pd throughout the coating. The standard coating contains anequal amount of Pd on conventional CeO₂/Al₂O₃ support. Samples are agedat 750° C., 24 hours, 10% steam/air prior to testing. Testing isperformed at 450° C., space velocity 110,000 h⁻¹ with CO injection at 1Hz and lambda swing at 0.98 to 1.08; NO=500 ppm, HC (C₃H₆/C₃H₈)=1800ppmC, CO₂=10%, H₂O=7%, CO/O₂ varies base on lambda. The amount COexiting the coated monoliths is determined by FTIR infraredspectroscopy. Results are in FIG. 2. It is seen the inventive coatedmonolith provides outstanding results vs. a comparative coating.

1. A catalytic article comprising: a substrate having a catalyticcoating thereon, the catalytic coating comprising: a catalytic layerhaving a thickness, an inner surface proximate to the substrate and anouter surface distal to the substrate, wherein the catalytic layercomprises a noble metal component on support particles and wherein theconcentration of the noble metal component towards the outer surface isgreater than the concentration towards the inner surface.
 2. A catalyticarticle according to claim 1, wherein at least 50 wt % of the noblemetal component resides in the outer one fifth of the thickness of thecatalytic layer.
 3. A catalytic article according to claim 1, wherein atleast 70 wt % of the noble metal component resides in the outer one halfof the thickness of the catalytic layer.
 4. A catalytic articleaccording to claim 1, wherein where ≥80 wt % of the noble metal residesin the outer 20% of the thickness of the catalytic layer.
 5. A catalyticarticle according to claim 1, wherein the noble metal is palladium orplatinum.
 6. A catalytic article according to claim 1, wherein thesupport particles comprise a refractory metal oxide selected from thegroup consisting of alumina, zirconia, titania, ceria, manganese oxide,zirconia-alumina, ceria-zirconia, ceria-alumina, lanthana-alumina,baria-alumina, silica, silica-alumina and combinations thereof.
 7. Acatalytic article according to claim 1, wherein the support particlescomprise oxides of cerium, zirconium, neodymium, praseodymia, lanthanum,yttrium or combinations thereof.
 8. A catalytic article according toclaim 1, wherein the catalytic layer comprises ceria, alumina andzirconia.
 9. A catalytic article according to claim 1, wherein the noblemetal is present from about 5 g/ft³ to about 100 g/ft³, based on thevolume of the substrate.
 10. A catalytic article according to claim 1,wherein the catalytic layer contains support particles having a bimodalparticle size distribution comprising micron-scaled particles andnano-scaled particles.
 11. An exhaust gas treatment system comprising acatalytic article according to claim 1 in fluid communication with anddownstream of an internal combustion engine.
 12. A method of treating anexhaust stream of an internal combustion engine comprising contactingthe exhaust stream with the catalytic article according to claim
 1. 13.A method of making a catalytic article comprising: providing a firstmixture comprising micron-scaled support particles and applying thefirst mixture to a substrate to form a micro-particle layer; providing asecond mixture comprising nano-scaled support particles and a noblemetal component having an initial pH and applying the second mixture tothe micro-particle layer; and calcining the substrate.
 14. The methodaccording to claim 13, wherein the micron-scaled particles have anaverage particle size from about 1 to about 11 microns and thenano-scaled particles have an average particle size of ≤950 nm.
 15. Themethod according to claim 13, wherein the second mixture is a sol or acolloidal dispersion.
 16. The method according to claim 13, furthercomprising adjusting the initial pH of the second mixture to ≤6.
 17. Themethod according to claim 13, wherein the second mixture furthercomprises an organic dicarboxylic acid.
 18. The method according toclaim 13, wherein the first mixture further comprises micron-scaledoxygen storage component particles and/or the second mixture furthercomprises nano-scaled oxygen storage component particles.
 19. The methodaccording to claim 13, wherein the second mixture is a sol.
 20. Themethod according to claim 13, wherein the second mixture comprises azirconium sol, an aluminum sol and a cerium sol.